VU
meters measure the volume (intensity) of analog audio signals. They
display signal levels in volume units (VU), a measure of average volume
level. By definition, a volume indicator (VI) reading of 0 VU describes
a 600-ohm resistance in which 1 milliwatt (mW) of sine-wave power flows
at 1000 cycles per second (CPS). Most VU meters have a scale from -20
to +3, and rise and fall times of 300 milliseconds (ms). If a constant
sine-wave of 0 VU is applied, the meter will achieve 0 VU in 300 ms.
Readings on the VU scale are approximately log to the base 10 of the
power ratio referenced to that level. For a pure 100-Hz sine wave, 0 VU
equals 0 dBm or 0.775-V root means square (RMS). In practice, however,
VU meters contain an isolating resistor so that 0 VU equals +4 dBm for
a pure sine wave.

Selecting
audio meters requires an understanding of the differences
between VU meters and peak program meters (PPM). Whereas VU meters
display the average volume level of an audio signal, PPMs display peak
volume. The difference between the reading of a VU meter and a peak
program meter (PPM) is called the crest factor. VU meters and PPMs also
differ in terms of acceleration and deceleration rates (ballistics).
Whereas a VU meter takes 300 ms to stabilize when a 1 kHz steady-state
tone is introduced, a PPM stabilizes in just 10 ms. Because of their
ballistics, then, VU meters must sample the audio signal over a longer
period of time than audio meters such as PPMs.

VU
meters are designed to provide a very slow response, and are
optimized for perceived loudness instead of peak performance. In terms
of their internal circuitry, they contain a single diode, a selenium
bridge rectifier, or a germanium diode bridge. Although active
rectification is used to keep the diode voltage-drop from causing large
inaccuracies, amplification of the signal can reduce the overall
component count. To control the VU meter’s movement and minimize
overshoot, some devices include capacitors.

You
can replace the VU meters on the Topaz. Expensive replacement VU meters
are fromSifam, other VU meters
will work, look for ma meters with VU scale calibration to replace the
broken meters in your Topaz.

Replacing the lamps inside
the factory VU meters

This procedure is for replacing the lamps in the
factory VU meters
on the Soundtracs Topaz meterbridge.

1. Remove meterbridge from console. You will have to
take apart the meterbridge in order to replace the lamps.

2.
Remove all of the outer screws holding together the meterbridge's front
and rear panel. Once all of the front and rear external screws
are removed, the meterbridge's front and rear panels will seperate into
two parts rather
easily. Check to be sure you got the screws underneath the front LED
meters. No need to force or pry open the meterbridge once all of the
screws are removed.

looking at the meterbridge in it's two halves...

A.
The front panel, which will have mounted to it the LED meter
pcbs & the VU meter module.

B. The rear panel, the meterbridge's toroidal transformer
& the XLR terminal for the external transformer.

3.
Easy to follow from the transformer are the power lines,
terminated with molex plugs, which are connected to the VU
meter module.
Commit to memory the molex connections of your front and rear panel to
the VU module or take a photo if you need, you are going to need to
plug
it back together. Take note of the pin location for the connection
leading away from the VU module to the front panels LED pcb, this
specific molex only plugs in to two of the available four pins (see
pics). Because we will want to remove the VU module from the rear panel
of the meterbridge, unplug the molex connections from the VU meter
module.

note:
This is looking at the VU meter module upside down.

4.
Once you have unplugged the molex connections and removed
the four screw clamps
holding the VU module to the front panel of the
meterbridge, the VU module should come away
free from the front panel.

5. Place the VU module on a flat surface with the
meters facing you.

Wondering if this is the meter's
calibration or needle adjustment screw? "circled yellow"

6.
With just your hands you can easily remove the front plastic covers to
each of your VU meters on the module. If you want to be more careful
you can use a small screw driver and along the base of the plastic VU
meter covers you can find availble opennings to easily pry off the
plastic VU covers.

7.
With the plastic covers removed you can
easily see the Lamp bulb lying across the bottom of the meter,
right across the needle's bridge. There are two solder connections for
each bulb on each meter.

8. Heat up your soldering iron to
remove the old factory Lamp from these solder connections. A light
touch with your iron should be effecient enough to warm up the solder
at the
connection and free the leg pins of the Lamp. Each Lamp has two
connections at the base of the meter.

In
addition to basic soldering skills, a soldering iron, and solder, you
might also need a solder wick or pump for removing solder, a small pair
of wire cutters, a magnifying glass.

9.
Use the old Lamp bulb pins as a guide to how much length you will need
from the bulb's leg pins in order to solder the bulb between the two
connection points. Also, you might observe how the factory bent the
connection wires of the bulb and where the bulb rests on the meter.
(see pics)

10.
When you solder in the bulbs, you can take two approaches. First, use a
little extra solder on the tip of your iron and just solder the lamp
pins right down into the existing factory solder. Seven times out of
ten this will work fine and your bulbs will illuminate solid. Though
sometimes this may result in an intermittant bulb, turning on and off,
or flickering once all put back together. The old solder is causing a
bad non continuos connection. So your second approach may be to use a
desoldering wick and pull some of the old factory solder off the vu
meter's connections, and thus being able to solder directly to the
terminal. Both procedures were called for when I did my replacement.
The yellow fuzzy plastic, carpetting the bottom half of the VU meter
didn't help for a clean blulb replacement.

11.
Use you DVM to do a continuity test from the far side of each of your
solder connections to the bulb, to feel confident that your solder
joints are solid before reassembling your meterbridge to enjoy your
new replacement bulbs.

12. Remount the VU meter module
to the front panel, correctly plug back up your molex connections
&
bolt back together your meterbridge. All Done.

How to adjust,
calibrate & test your VU metersThe meter's adjustment screwfitted into the bottom center
"solid black plastic area" of the vu meter's plastic cover the
vu meter is for setting zero. If your
needle seems a bit off zero when at rest, you can use a flat head screw
driver to correctly adjust the needle.

to correctly adjust
your VU meter...

You
will need a 400~1KHz constant tone (I use a signal generator) and an
accurate meter good at those frequencies. Your garden variety DVM is
very little use past 120 Hz. A VTVM, vacuum tube volt meter, would
certainly be ideal.

To
adjust VU meter, try running the same sine wave into two channels via Y
cable. Use a sine wave no higher than 1KHz and adjust the VU meters to
zero to match.

If
your DAW is wired into two of your mixer's channels...

set the pans to center, faders to zero, play these audio
test samplesand adjust your VUs
accordingly.

A
VU meter indicates the signal level being sent from the mixer to the
next device. A VU meter can be calibrated to any reference level
desired. In the United States, the most common reference level is 0VU =
+4dbm. That is, when the VU meter needle is lined up with 0 reading on
the meter face, the audio signal at the output of the mixer has a level
of +4dbm. Please note that the 0 indication on any VU meter has no
pre-determined value. 0 VU is simply a point of reference.What does +4dbm equal in
volts?

+4dbm equals a signal
voltage of 1.23 volts measured across a 600 ohm load.What is required to
calibrate a VU meter for a reference level of 0VU = +4dbm?

A multimeter that can accurately measure audio signal levels at 1kHz. A
good multimeter is required. Do not use inexpensive multimeters as the
calibration will not be accurate.

A 600 ohm
load resistor. Create a 600 ohm resistor by using one 270 ohm resistor
and one 330 ohm resistor. Solder the lead of one resistor to one lead
of the other. This puts the resistors in series and their values add
together to create a "new" resistor with a value of 600 ohms.

An XLR
female connector to mate with the mixer's male XLR output
connector. Remove the outer metal shell of the female XLR connector as
it will not be used. Solder one lead of the 600 ohm resistor to pin 2
of the female XLR connector. Solder the other lead of the 600 ohm
resistor to pin 3 of the female XLR connector.

Use a set of insulated alligator clip test leads to connect the
multimeter to the 600 ohm load.

Here are the steps for VU
meter calibration.

1. Set the XLR output level of the mixer
to LINE.
2. Connect the female XLR connector with
the 600 ohm load to the mixer's XLR output.
3. Using the alligator clips, connect one multimeter probe to
pin 2 of the female XLR. Connect
the other
multimeter probe to pin 3 of
the female XLR.
4. Power up the multimeter and set it to
read AC voltage.
5. Power up the mixer.
6. Turn on the mixer's 1kHz tone
oscillator, a signal generator or
line feed : sampled
recorded 1kHz tone (see samples above)
7. Turn up the mixer's Master control until the multimeter reads 1.23
volts.
Note: on some mixers you
may also have to turn up the level
control for input #1.
In that case, set the
Master to the middle of its range and turn up input #1 until
the multimeter reads 1.23
volts.
8. With the multimeter reading 1.23 volts (or as close as possible),
adjust
the mixer's VU
calibration
control
until the meter reads 0 VU.
See your mixer's User Guide to find the location of the VU
calibration
adjustments.
9. If you have a stereo mixer, do
this same calibration for the Left output and the Right output.
Making certain
that the channel pan pots are in the center
position.

Other common calibration
levels besides +4dbm are...

+8dbm = 1.95 volts; 0dbm
= 0.775 volts

Question: What is Unity
Gain?

A
reader writes: "I keep hearing about the idea of "unity gain" when
talking about microphone preamps and my recording mixer. What is unity
gain, and how does it help?"

Answer: When we talk about "gain",
we're talking about one device's ability to take the lower level of one
signal and bring it to a higher voltage level. A great example of this
is a microphone preamp; a preamp applies gain to amplify the signal
coming from a microphone.

In the idea of unity gain, the input
and the output between two devices are the same level. That's to say,
when a microphone is outputting 0db, a mixer will also be outputting
0db worth of signal. Unity gain is established by calibrating two
pieces of equipment to talk at the same level.

Frequently, the
best way to set unity gain is to adjust a microphone or line signal
gain to 0db, measured both at the preamp and output stage, and then
match that 0db level simultaneously on the input of the second piece of
equipment -- whether an amplifier, recording software, or mixer.

Unity
gain is useful for several reasons. First, it gives significantly
cleaner and non-distorted signal. Microphones will sound much fuller
and with much better dynamic range. There will also be a much easier
time applying outboard effects, as most effects units are set to accept
a unity signal.

If you're mixing sound live, getting proper gain
staging is important. You'll also nearly eliminate major feedback
problems, as a proper unity gain allows optimal gain-before-feedback.
Keep in mind, the more gain you can get cleanly and without distortion,
the better your mixes will sound!

Some
very educated answers are made in this forum conversation. I have
paisted here the more learned text from the link conversation above. In
the original conversation the user has a Rosetta 200 in their setup, in
my editted text below I replaced this with DI ( digital interface ).
The Apogee Rosetta 200 was a 2-channel 192kHz 24-bit AD/DA Converter.
This theory should work for any DI device in the recording chain, and I
don't own one of these DIs, so it helps me to think sometimes if I edit
out the non-existent equipment.

Ok,
here is an opportunity to clarify this dB business. dB is an
abbreviation for decibels. Decibels are a logarithmic measurement,
which means they are not a linear measurement. 1+1=2, but 1dB+1dB !=
(the "!=" means does not equal) 2dB. Decibel measurements are always
referenced to a nominal operating level, which is defined as 0 dB. Not
only are decibel measurements referenced to a nominal operating level,
but they are also referenced to another measurement! To truly
understand a measurement expressed in dB, you need to know...

1.
The measurement that is being referenced to.2.
What the 0dB value of the referenced measurement is.

+4dBu
(remember, capitalization matters) means that a device is operating at
4dB above (hence the + sign) 0dBu, which was defined above as being
0.775VRMS. The u was defined as
meaning the high impedance circuit has no load connected. So if 0dBu =
0.775 VRMS, then what voltage is represented by +4dBu? This is where
the log and anti-log keys on your calculator become useful. The formula
for calculating voltages with logs is dBu = 20 log (base 10) Vo/Vi
where dBu is the reference value, Vo is the output voltage, and Vi is
the input voltage. We know that 0 dBu = 0.775VRMS, so to determine what
voltage corresponds to +4dBu, we calculate it as

Now
we take the anti-log of .2 This can be thought of as raising 10 to the
power of .2.

10
anti-log .2 = Vo/Vi1.585
= Vo/Vi

Now,
we know the input voltage is 0.775VRMS, so

1.585
= Vo/0.7751.585*0.775
= Vo1.228VRMS
= Vo

We
have just calculated that audio equipment whose nominal operating level
is specified as +4dBu operates at a nominal voltage of 1.228VRMS! stike
cooge What are the implications of this? Well, besides learning some
math, you now know that if your audio interface outputs, digital I/O
(DI BOX), and Topaz inputs are all specified to operate at a nominal
level of +4dBu that you could use a voltmeter at each point in the
chain to verify that everything is operating at unity gain by checking
to see that the voltage is 1.228VRMS.

The
simple procedure from the lecture...

1.
Play a 1kHz sine tone whose dbFS level you want to represent 0 dB VU.
Make certain that all of your faders in your DAW are at unity (0 dB
gain) with no other processing.

2.
In this case you want to calibrate your DI BOX.
So route the sine tone output of his DAW into the DI. Then
calibrate the DI so that its meters indicate the same level as the sine
tone is set to in the DAW.

3.
Then set the faders on the Topaz at unity (0 dB gain) and adjusts the
trim until the Topaz meters indicate the same level as the sine tone is
set to in the DAW.

4.
If he wants to be certain he is operating at unity gain throughout the
above chain, he can use a voltmeter to measure his DAW's output and his
DI's output to be 1.228VRMS.